In the third generation partnership project (3GPP), heterogeneous network deployments have been defined as deployments where low-power nodes of different transmit powers are placed throughout a macro-cell (cells served by a radio base station (RBS) transmitting with higher power) layout, implying also non-uniform traffic distribution. Such deployments are, for example, effective for capacity extension in certain areas, so-called traffic hotspots, i.e. small geographical areas with a higher user density and/or higher traffic intensity where installation of pico nodes (a type of low-power radio base stations) can be considered to enhance performance. Heterogeneous deployments may also be viewed as a way of densifying networks to adopt for the traffic needs and the environment. However, heterogeneous deployments also bring challenges for which the network has to be prepared to ensure efficient network operation and improved user experience. Some challenges are related to the increased interference in the attempt to increase small cells associated with low-power nodes, aka cell range expansion; the other challenges are related to potentially high interference in uplink due to a mix of large and small cells.
According to 3GPP, heterogeneous deployments consist of deployments where low power nodes are placed throughout a macro-cell layout. The interference characteristics in a heterogeneous deployment can be significantly different than in a homogeneous deployment, in downlink or uplink or both.
Examples hereof with closed subscriber group (CSG) cells are given in FIG. 1a illustrating a communication network 10 comprising a macro cell 14 served by a macro node 13 and a plurality of pico/femto cells 15 (e.g., the pico/femto cells 15a, 15b, and 15c depicted in FIG. 1a) served by pico/femto nodes 12. In case (a), a macro user 11a (a user equipment (UE) connected to the macro RBS 13) with no access to the CSG cell 15a (served by a low-power RBS 12a) will be interfered by the low-power RBS 12a (e.g. a Home Node B, HeNB), in case (b) a macro user 11b causes severe interference towards the HeNB 12b and in case (c), a CSG user 11c served by CGS HeNB 12b is interfered by another CSG HeNB 12c. Heterogeneous deployments, however, are not limited to those with CSG involved.
Another example is illustrated in FIG. 1b, where the need for enhanced intercell interference cancellation (ICIC) techniques for downlink (DL) is particularly crucial when the cell assignment rule diverges from the reference signal received power (RSRP) based approach, e.g. towards pathloss- or pathgain-based approach, sometimes also referred to as the cell range expansion when adopted for cells with a transmit power lower than neighbor cells. In FIG. 1b, the cell range expansion of a pico cell 15 is implemented by means of a parameter Δ. The pico cell 15 is expanded without increasing its power, just by changing the reselection threshold, e.g., UE 11 selects cell of the pico RBS 12 as the serving cell when RSRPpico+Δ≧RSRPmacro, where RSRPmacro is the received signal strength measured for the cell 14 of the macro RBS 13 and RSRPpico is the signal strength measured for the cell of the pico RBS 12.
There are transmit patterns and measurement patterns for evolved ICIC (eICIC), i.e. ICIC in 3GPP long term evolution (LTE) standards. To facilitate measurements in the extended cell range, i.e., where high interference is expected, the standard specifies Almost Blank Subframe (ABS) patterns for evolved Node B (eNodeB or eNB) and restricted measurement patterns for user equipments (UEs). A pattern that can be configured for eICIC is a bit string indicating restricted and unrestricted subframes characterized by a length and periodicity, which are different for frequency division duplex (FDD) and time division duplex (TDD) (40 subframes for FDD and 20, 60 or 70 subframes for TDD). Only downlink (DL) patterns have been so far specified for interference coordination in 3GPP, although patterns for uplink (UL) interference coordination (IC) are also known.
An ABS pattern is a transmit pattern of an RBS 13 transmitting radio signals. The ABS pattern is cell-specific and may be different from the restricted measurement patterns signaled to the UEs 11. In a general case, ABS are low-power and/or low-transmission activity subframes. ABS patterns may be exchanged between eNodeBs via the X2 interface, but the patterns are not signaled to the UE, unlike the restricted measurement patterns.
Restricted measurement patterns (i.e. “time domain resource restriction patterns” [3GPP technical specification (TS) 36.331]) are configured to indicate to the UE 11 a subset of subframes for performing measurements, typically in lower interference conditions, where the interference may be reduced e.g. by means of configuring multicast broadcast single frequency network (MBSFN) subframes or ABS subframes of interfering eNodeBs 13.
Restricted measurement patterns may, however, also be configured for UEs 11 with good interference conditions, i.e., receiving a measurement pattern may not be necessarily an indication of expected poor signal quality. For example, a measurement pattern may be configured for a UE in the cell range expansion zone where typically high interference is expected, but a measurement pattern may also be configured for UEs located close to the serving base station 12 where the signal quality is typically good which may be for the purpose of enabling a higher-rank transmission modes (e.g., rank-two transmissions using multiple input multiple output (MIMO) transmission).
Restricted measurement patterns are in general UE-specific, although such patterns may be broadcasted or multicasted. Three patterns are currently specified in the LTE standard for enabling restricted measurements:                Serving-cell pattern for radio link monitoring (RLM) and radio resource management (RRM) measurements,        Neighbor-cell pattern for RRM measurements,        Serving-cell pattern for channel state information (CSI) measurements.        
Transmit patterns and measurement patterns are means for coordinating inter-cell interference in wireless network and improve measurement performance. Alternatively or in addition to inter-cell interference coordination techniques, measurement performance may also be improved by using more advanced receiver techniques, e.g., interference suppression or interference cancellation techniques.
The UE 11 is generally aware about the serving cell(s) 12 configuration. However, the UE is not only receiving/sending data and performing measurements in the serving cell(s), it may also move, for which the information about neighbor cells may be helpful for mobility decisions, or the network or the network and/or the UE may also perform different radio resource management (RRM) tasks and hence measurements in neighbor cells may be needed. In LTE standard Release 10 (Rel-10), the UE may receive the aggregate neighbor cell information, e.g., an indication on whether all neighbor cells use the same MBSFN configuration as the primary cell (PCell).
Neighbor cells lists have been mandatory for mobility and RRM purpose in earlier networks, e.g., Universal Terrestrial Radio Access (UTRA). However, such lists (comprising e.g. neighbor cell identities) are optional in LTE, and the UE has to meet the same requirements, irrespective of whether the neighbor cell information is provided to the UE or not.
Further, the UE 11 also receives interference from neighbor cells 13 and the UE receiver may benefit from the knowledge about the interference character (e.g., when the interfering signal occurs and where in the frequency dimension). In LTE Rel-10, to enable eICIC, the UE may receive measurement patterns via its serving cell or PCell (if channel aggregation is used), as described above, for measurements in the serving cell or neighbor cells. In the latter case, only one measurement pattern is provided per frequency for multiple measurement cells, together with the list of physical cell identities (PCIs). In Rel-11, the UE should be capable to deal with even higher interference and hence even more network assistance may be needed for the UE. For example, it has been proposed that the UE should be provided the information about the number of common reference signal (CRS) ports and the MBSFN configuration of at least some interfering cells.
In LTE Rel-10, enhanced interference coordination techniques have been developed to mitigate potentially high interference, e.g., in a cell range expansion zone, while providing the UE with time-domain measurement restriction information. Further, for LTE Rel-11, advanced receivers based on Minimum Mean Square Error-Interference Rejection Combining (MMSE-IRC) with several covariance estimation techniques and interference-cancellation-capable receivers are being studied. One example of such IC receivers are receivers capable of removing interference from known signals (for instance CRSs transmitted in all macro cells in all sub frames). The CRS then interfere also in blank sub frames, i.e. in sub frames where the macro cell does not transmit data, in the particular resource elements (REs) where the CRSs are transmitted. An advanced IC receiver can in principle estimate the received signal from the macro cell at REs where known CRSs are transmitted and then subtract that interference in these REs. Then the decoding of data from the pico cell RBS is made.
US 2011/0267937 discloses a method to enhance coverage and/or throughput in a heterogeneous wireless network, including detecting interference between a neighbouring cell and a serving cell. The method also includes cancelling the interference using an adaptive technique based on whether the interference has colliding Common Reference Signal (CRS) tones. According to this document, a UE may adaptively apply a particular CRS cancellation approach for a given scenario. Thus, the UE may choose one of the algorithms depending on the cell IDs the UE sees, where cell IDs indicate whether the interference includes colliding CRS tones. One algorithm is chosen if the cell ID indicates that the interfering CRS tones are colliding, and another algorithm is chosen if the cell ID indicates that the interfering CRS tones are not colliding.